Stents are small, intricately cut tubes, generally made of materials such as stainless steel. Cardiovascular stents are permanently placed in a blood vessel to act as scaffolding to keep an occluded artery open. In use, cardiovascular stents are inserted into the artery on a catheter and are typically deployed by expanding a very small balloon at the end of the catheter upon which the stent is mounted.
Cardiovascular stents must meet stringent requirements to work properly. If the stent contains any rough or sharp edges, it will damage blood cells or the blood vessel in which it is inserted. This can lead to further atherosclerotic plaquing, emboli or thrombi, and result in potentially life threatening situations.
Lasers are typically used to cut stents. This process, while highly precise, can occasionally produce defective parts. Stents tend to be small, with diameters approximating 1 mm. After processing, the individual cut features on a stent range from 50 to 200 microns in size. Accordingly, small changes in manufacturing process parameters such as laser power, tubing diameter, or mechanical jitter can cause defects. Such defects may include an out of tolerance feature size or a malformed feature.
Since stents are used in the heart and other critical areas of blood flow, a failure in the function of the stent could be life threatening. Thus, manufacturers of stents typically employ 100% inspection procedures. A human operator utilizing a 50× optical power stereomicroscope typically inspects for visual defects. Dimensional inspection is typically done by a human operator utilizing a profile projector. Alternatively, this inspection can be done automatically by utilizing a vision system.
The problems associated with either the manual or the automatic approaches to inspection are numerous. First, human error makes visual inspection of products less than completely effective. In addition, such manual inspection is relatively slow and thus a relatively costly aspect of the manufacturing process. Furthermore, the pass/fail criteria of the profile projector using overlays, as is typically employed in manual inspection, does not generally provide any numeric dimensional data that would otherwise be useful for process control.
Stents are typically highly polished and have a very convoluted geometry with many intricate shapes. One ideal approach in inspection work is to compare the as-found geometry to the nominal or CAD model. While there are many practical commercial embodiments of systems to make this sort of inspection on generally rigid parts, these systems are not practical for the flexible stents.
CAD models are typically only available that represent the part ready for final inspection. It would be helpful for process control to have a means of creating a CAD model at different steps along the production cycle and have that model available for in-process inspection.
A means to image the highly contoured features that are part of the stent geometry has not been found. While methods are known for imaging the outside and the inside of stents, there has not been developed a practical method for imaging the sidewalls of these cut metal tubes, especially when the spacing between the cuts are narrow. In addition, a means has not been demonstrated to utilize a line scan camera with stents of any geometry except cylindrical.